Checking with uptimerobot would only tell you about your ping to the uptimerobot servers and different nodes may even be checked by different servers. So you can’t really use that.
We should probably start out by checking whether latency is actually causing transfers to be cancelled in long tail cancelation. You can use the success rate scripts for that, but you would need a bit of history in logs.
If those success rates look good then it’s not latency.